Voice coil bobbins for cutters - Test phase 1

[markrob]

Hi,

Looks like you are in good shape to experiment. Keep in mind that you don't want to wind such that the DC resistance equals 8 ohms. In general, the DC resistance will always be lower. I would shoot for 4-6 ohms DC resistance. Ideally, you want the largest total wire length that will give you that 4-6 ohms range. I wouldn't get too hung up on this. Give it your best shot and test the sensitivity at 1Khz. If it takes about 2.8Vrms (1 watt into 8 ohms) to cut a 5 cm/sec level, you are in the right ballpark. The great thing about having the ability to wind your own coils is that you can easily experiment. I'm sure there are sources on the net that would help you come up with the correct winding given wire gauge, DC resistance, wire length, and bobbin dimensions, but you should be able to live with a less than optimized coil.

Mark

[L.K.R.A]

Hi Mark,

I've been doing a bit of reading and a bit of math over the past 2 days...

Agreed that the dc should be at about 6 to get an ac of close to 8

I've also noticed that there's slight differences in the wiki chart and the chart I've been using - not major, one measurement worked out to have a difference of about 15 cm...

So as an approximate from the wiki chart...

Using 31 gauge I will require about 46 feet to obtain 6 dc - I will split this over 2 bobbins for the presto head = about 23 feet each bobbin...

Using 34 gauge I will require about 23 & 1/4 feet to obtain 6 dc - this will be used on the one bobbin alone...

I'm guessing that the differences in the charts can be attributed to different testing environments and/or different composition of wire materials tested so I will just assume the measurements as approximates until my stock arrives and I get winding + measure them...

We also must take into account that the measurements on the charts may have been taken on bare wire and not enamelled so the measurements could be different again...

"The cloth wire has arrived, the enamelled wire and varnish are on their way and I've just paid for the coil winder so that'll be on it's way shortly" - Next is that meter with test tone on ebay that I pictured earlier...

without having a tone generator and all the other fancy gear to do full measurement of impedance & response - that little meter will do me for now...

[markrob]

Hi,

One other item worth mentioning is that the Presto heads have specified series resistance added to allow for the correct low frequency turnover. IIRC about 2 ohms for the 8 ohm head. So, you might only want 2-4 ohms total resistance with the two bobbins wound.

Are you sure you'll be able to get that much wire in each bobbin? Remember, you can reach the target as far as ohms go with many different gauges of wire. But for any given gauge, you may not be able to fit it into the available bobbin space.

I went back and checked my notes. I had a 1C head factory wound for 8 ohms (the 1D only came in a 16 ohm winding IIRC). I measured those coils at 8'2" with 29 gauge wire. That was my best guess based on the diameter I measured. That measured .84 ohms per bobbin. Coupled with a series 2 ohm resistor, that puts you near to 4 ohms.

A 500 ohm 1D head had 120' of #37Ga wire.

Mark

[emidisc]

Interesting thread!
After a lot of practice I can wind coils for my cutter heads (x2) BSR with little problem and initially they sound pretty good but do eventually fade then blow so what would you guys suggest for a suitable adhesive for the coils,
The problem I have is that they are "saddle" coils rather than round or cylindrical so unless the form is perfect they won't fit in position after the adhesive is set.
For anybody unfamiliar with saddle coils they are used on the Grampian 1D
Emidisc

[markrob]

Interesting thread!
After a lot of practice I can wind coils for my cutter heads (x2) BSR with little problem and initially they sound pretty good but do eventually fade then blow so what would you guys suggest for a suitable adhesive for the coils,
The problem I have is that they are "saddle" coils rather than round or cylindrical so unless the form is perfect they won't fit in position after the adhesive is set.
For anybody unfamiliar with saddle coils they are used on the Grampian 1D
Emidisc

Hi,

I didn't use any potting varnish on the heads I re-wound. I did put a bit of Duco cement (a nitrocellulose based glue) on the outside of the coil along with some cut strips of masking tape to keep it from unraveling. It would have been better to pot the coil, but I wanted to be sure it worked out first before locking things down.

I'm interested in you experiences winding a saddle style coil. (I have a BBC Grampian type B1/AGU here). Do you wind the coil on a cylindrical mandrel and then bend to shape with a form or do you wind in the final shape to start? Do you also add any feedback winding along with the drive when you wind the coil? My Grampian has some from of tape covering the coil. I'm not sure if its potted as well as I have never had the need to dissect it.

Mark

[emidisc]

Hi Mark,
I have tried various ways of winding saddle coils but the most successful method for me was simply winding a cylindrical coil that is oversized enough to allow for the later manipulation into the saddle,
I too use masking tape cut into thin strips with a little super glue on the ends of the tape but initially to get the coils tight I use small cable ties in 2 or 3 positions and then the masking tape then remove the cable ties when I'm happy with the coil,
Next I offer up the coil to the poll pieces gently bending when required, I've yet to try including a feedback winding but I have very little space for more wire?
I don't get to hung up over the resistance figure I just aim for somewhere between 6 and 8 Ohms DC
I would like to put some sort of damping between the pole pieces and the armature was thinking along the lines of oil soaked paper?

Emidisc

[L.K.R.A]

Hi,

One other item worth mentioning is that the Presto heads have specified series resistance added to allow for the correct low frequency turnover. IIRC about 2 ohms for the 8 ohm head. So, you might only want 2-4 ohms total resistance with the two bobbins wound.

Are you sure you'll be able to get that much wire in each bobbin? Remember, you can reach the target as far as ohms go with many different gauges of wire. But for any given gauge, you may not be able to fit it into the available bobbin space.

I went back and checked my notes. I had a 1C head factory wound for 8 ohms (the 1D only came in a 16 ohm winding IIRC). I measured those coils at 8'2" with 29 gauge wire. That was my best guess based on the diameter I measured. That measured .84 ohms per bobbin. Coupled with a series 2 ohm resistor, that puts you near to 4 ohms.

A 500 ohm 1D head had 120' of #37Ga wire.

Mark

As posted earlier, I've been able to replicate the single bobbin with the 29 gauge up around the 0.8 area (without a digital mutli I don't know the exact figure) so when its in series with another bobbin and with a 2 ohm resistor = Done

As for the other gauges and lengths - "they are an approximate and I won't know if they'll fit for sure until I get winding"

Also looking at getting 32 and 36 gauge so I've got a range of wire to test with

"Just waiting for the new wire and coil winder to arrive and then we'll see what happens"

[Angus McCarthy]

I also own a presto 1C which will be next on the rebuild list if the bobbins hold up and pass testing...

If I don't get to rebuilding my 1D coils before that, I'll gladly throw some cash your way for a new pair.
Now to figure out the weird shape of the 5C coil...

[aussie cutter]

L.K.R.A has been successful with the manufacture of the voice coil bobbins....

Check out their facebook page...

https://www.facebook.com/LatheKutRecordsAustralia

The 15 ohm Universal cutter that was being worked on is finished and there's photos and a video of it being tested...

[aussie cutter]

If you are interested in getting 1 or 2 bobbins for your cutter - Contact L.K.R.A via P.M, E-mail or Facebook